Question from Rick Beales: We're planning a new build-out. We have always just used 208V/30A circuits, but our new cabinet plans require up to 7kW. What are the trade-offs between just adding a second pair of single-phase circuits to our standard plans and going to 3-phase circuits for new builds?
Engineer reply: As you point out, the main reason to provide 3-phase power is to increase the amount of power available. A single phase 208-volt power run requires three conductors, two "hot" lines and a ground; and can provide a continuous power of 5.0kW for a 30-amp circuit. A three-phase 208-volt power run requires four conductors, three "hot" lines and a ground; and can provide a continuous power of 8.6kW. So, by increasing the number of conductors by 33% (from 3 to 4), power delivered increases by 73%. Being that your 7kW requirement is still below the 8.6kW rating of the 3-phase circuit, you are most likely to find after a full cost analysis that the single (redundant) pair of 3-phase circuits will be much less expensive than two pairs of single-phase circuits.
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